Aluminum-copper-magnesium powder metallurgy

ABSTRACT

Sintered compacts of aluminum-base powder containing copper and magnesium are obtained with superior mechanical properties by using as the copper component a non-leafing type copper flake containing on its surface from about 0.03 to a maximum of about 0.05 weight percent of a &#39;&#39;&#39;&#39;leafing&#39;&#39;&#39;&#39; component such as stearic acid or oleic acid, and the like.

United States Patent Storchheim Feb. 19, 1974 [54] ALUMINUM-COPPER-MAGNESIUM 3,333,950 8/1967 Hill 75/0.5 R

POWDER METALLURGY 3,357,818 12/1967 Findeisen 75/05 R 3,401,033 9/1966 Bartz 75/0.5 R [76] Inventor: Samuel. Storchheim, 104-40 Queens Blvd" Forest Hlns Primary ExaminerW. W. Stallard [22] Filed: May 17,1972 [21] App]. No.: 254,013 [57] ABSTRACT 52 us. 01. 75/05 R, 264/111 ing coppr and magnesium are Obtained with up 51 1111. c1. B221 3/00, B22f 9/00 mechanical properties by using as the pp p [58] Field of Search 75/05 R; 264/111 mm a non-leafing yp pp flake containing on its surface from about 0.03 to a maximum of about 0.05 5 References Cited weight percent of a leafing component such as ste- UNITED STATES PATENTS aric acid or oleic acid, and the like. 3,250,838 5/1966 Bartoszak .Q 264/111 4 Claims, 3 Drawing Figures psi at 2 000 I I 24 hr. mix) LUlfimure Tensile Strength (77 E a. 0 29,000 4; [L c" (To 3.9% 28,000 0i 24hrs.) Q 1 U I -Elongciion g G 27,000

Sintered compacts of aluminum-base powder contain- Mixlng Time, Hours PATENTED FEB 1 9 1914 same, of 3 FIG. 1

(To 30,000 K psi at 32,000 I 24hr. mix) kUlflmoie Tensile Strength E 0) CL 8 29,000 0 0 0 n. m C o O D (T0 3.9 /o 2; 20,000 tifll fi) 0 o .3 -E|0ng0i'|0n g 5 Mixing Time, Hours PATENTEOFEB! 9 m4 SHEET 0F 3 352.: WOW x 96m 9.641 o=oE w bc E wocmh o e '6 VI m c w m h F O M V n m e 0 B .n e .4 L

3 0 w o o o. w R m $230 wS o TvCwm 622: w

Mixing Time, Hours .PAIENIEB FEB I 9 I974 SHEET 3 BF 3 FIG. 3

wohw 62 5 3 0965 595 4 Mixin Time, Hours ALUMINUM-COPPER-MAGNESIUM POWDER METALLURGY This invention relates to aluminum powder metallurgy and, more particularly, to aluminum-coppermagnesium powder mixtures which yield sintered compacts of improved mechanical properties.

It is old in the art of aluminum powder metallurgy to increase the strength of the final sintered compact by blending copper powder with the aluminum powder prior to compacting and sintering. In general, effective amounts of added copper powder have ranged from about l to about percent by weight of the aluminum powder and have yielded sintered compacts having a typical maximum tensile strength of about 26,000 psi and an elongation of about 2 percent when using about 4 percent by weight of copper.

The copper powder for producing high strength products has been in the form of both massive and flake-like particles. The flake-shaped particles, when used, have been of the leafing type which, when suspended in a liquid vehicle and applied as a lacquer or paint, arrange themselves in planes parallel to the surface to which they are applied. In the case of aluminum powder metallurgy, the advantage of using leafing-type copper flake is that the surface of the flake contains a sufficient quantity of stearic acid, which produces the leafing" property, to promote mixing of the copper with the aluminum powder and to act as a lubricant facilitating compaction of the resulting aluminum-copper powder mixture. In general, the leafing type copper flake powders contain about 0.5 to 2 percent by weight of stearic acid or oleic acid on their surfaces, as described in U. 5. Pat. No. 3,333,950, although commercially available leafing copper powders generally contain about I to-2 percent by weight of stearic or oleic acid. Commercially available non-leafing copper flake powders usually contain from about 0.13 to 0.25 percent by weight of stearic acid added as a lubricant in the milling process required to convert massive copper particles into flakes, and when these nonleafing copper flake powders are added to aluminum powder they totally resist distribution through the aluminum powder even after 24 hours of continuous mixmg.

l have now discovered that when non-leafingcopper flake powder contains from about 0.03 weight percent up to a maximum of about 0.05 percent weight percent of a leafing-type lubricant such as stearic acid or oleic acid on its surface, it visibly mixes readily with aluminum powder and, when mixed for a critical minimum of at least about one-half hour, yields sintered compacts of significantly higher tensile strength and of superior surface appearance to those aluminum-copper sintered compacts obtainable heretofore. The aluminum powder further contains a conventional amount of magnesium such as to accelerate or activate sintering of a compact of the metal powder mixture. The mixture is free from added internally lubricating amount of any organic material.

The novel powder mixture of the invention can use virtuallyall grades of aluminum powder, the only limitation being that the finer the powder the poorer its flow rate for discharge into a die under commercial operating conditions. For example Alcoas aluminum powder No. 1220 (9.7 percent minus 325 mesh Tyler), No. 120 (35.6 percent minus 325 mesh) and No. 123

(89 percent minus 325 mesh) have been used successfully with the conditional qualification, as mentioned previously, that the coarser powders had better flow characteristics.

A small amount of magnesium in the powder mixture of the invention accelerates or activates sintering of the mixture by forming a relatively lower melting point eutectic with the aluminum. The amount of magnesium useful for this purpose ranges between about 0.2 to 2 weight percent of the aluminum powder. Within this range, amounts of magnesium from 0.3 to 0.6 weight percent of the aluminum are presently preferred. The magnesium can be added as magnesium metal powder or in the form of a magnesium alloy powder. When added as an alloy, it is advantageous to use as the alloying constituent one of the other components of the powder mixture, such as aluminum or copper (i.e., aluminum-magnesium and copper-magnesium alloy powders) but other alloying constituents which are advantageous for, or at least not inimicable to, the desired properties of the sintered compact can be used in conjunction with the magnesium component of the mixture.

The copper flake powder is used in the powder mixtures of the invention in amounts ranging from 2 to 6 weight percent of the aluminum component. Amounts of copper of at least about 2 percent are required to impart age-hardening characteristics to sintered compacts made from the mixture, and amounts of copper in excess of about 6 percent increase the strength and hardness of the sintered compacts at too much expense in loss of ductility.- An amount of copper powder about 4 percent by weight of the aluminum powder presently appears to give a generally optimum combination of physical and mechanical properties.

The copper flake powder used in the metal powder mixtures of the invention must be of the non-leafing type but must contain the so-called leafing organic coating in amount limited to between about 0.03 and 0.05 weight percent of the copper flake. Copper flake powders are produced by flattening conventional copper particles of massive shape in the presence of a lubricating or polishing organic compound such as stearic acid, oleic acid, zinc stearate, lithium stearate, and the like. In working with copper flake powders which, as pointed out in U. S. Pat. No. 3,333,950, have a density close to that of aluminum powder, I found that the leaflng copper flake powders containing between 0.5 and 2 weight percent of the organic lubricating compound did not mix into the aluminum powder but remained as red colored striations or waves in the mixture even after mixing for 24 hours in a double cone blender. Such leafing copper flakes were expected, on the other hand, to blend readily with the aluminum powder because of the relatively large amount of lubricant on the copper flake surfaces, but the lubricant did not function in this manner. Further experiments with non-leafing copper flake powder, which commercially ranges between about 0.13 to 0.25 percent by weight or organic lubricant but in some instances contained as little as 0.03 percent of the lubricant, disclosed that most of these powders similarly resisted intimate mixing with the aluminum powder even when the amount of surface organic polishing or lubricanting compound was as low as 0.13 percent by weight of the copper. Not until there was used copper flake containing as little as about 0.05 percent by weight of the organic material on its surface did the copper flake enter into intimate admixture with the aluminum, and at this level, i.e., at about 0.03 to 0.05 percent by weight of organic compound, the copper flake powder was dramatically optically blended with aluminum within five minutes of blending time.

Aluminum-copper-magnesium powder mixtures of the invention, characterized by the presence of the copper in non-leafing" flake form containing from about 0.03 to 0.05 percent by weight of organic flaking lubricant, are further characterized by the absence of any additional quantity of the same or similar conventional powder-metallurgy internal lubricants. By excluding such added lubricants, the compacted powder mixtures of the invention can be sintered in any furnace atmosphere ranging from oxidizing to neutral to reducing in nature. Thus, the compacted powder mixtures of the invention can be sintered in air, steam, smelting furnace gases containing a variety of proportions of carbon monoxide and carbon dioxide, hydrogen, nitrogen, cracked ammonia, etc.

A study of the mixing time used to obtain an intimate physical admixture of the powder mixture components has shown that mixing time has a definite effect on the mechanical properties of the aged sintered compacts made from the powder mixture. For example, a series of runs were made using a jar-rolling technique for 2,000 gram lots of a powder mixture composed of Alcoas No. 1202 aluminum powder, 0.6 percent by weight of minus 325 mesh (Tyler standard) heliumreduced magnesium powder, and 4 percent by weight of copper flake powder containing 0.03 percent by weight of stearic acid. Glass jars were filled to one-half their volume and a wire screen was inserted into them to break up any agglomerates of powder during rotation of the mixtures. Mixing time was varied from onehalf hour to twenty-four hours. Results obtained were as shown in FIG. 1 which is a plot of mechanical properties versus mixing time for 4 inches X /2 inch X /8 inch bars that were green pressed to 95 percent theoretical density and then air sintered at l,l F. in a furnace through which they pass at a 2 /2 inches per minute belt speed (four minutes at temperature) with the furnace atmosphere consisting of ambient air. As

- can be seen, the curve for U.T.S. rises, peaks and then starts to slowly descend. Elongations followed the same pattern but did not drop with extended mixing time, namely, twenty-four hours. A mixing time of one hour minimum appeared to be required for this type of blending apparatus while optimum mechanical property values occurred after approximately three to four hours of mixing time.

Other properties likewise varied with the blend mixing time as indicated in FIGS. 2 and 3. FIG. 2 shows that the number of bend-to-fracture peaks to a maximum value at six hours mixing time while the difference in sintered lengths among the 4 inch long bars were at a minimum at the six hours mixing time. FIG. 3 shows an approximately 20 percent drop in growth (which occurs in air sintering) over the 4 inch lengths following sintering using the six hour mixing time blend.

A scale-up of the foregoing tests was made using a five cubic foot double-cone blender in which 250 pound batches of the same powder mixture were mixed for times varying from fifteen minutes through six hours. The findings from these tests were that more thorough mixing was encountered in a shorter time with the large production scale apparatus than for the small scale roller-jar combination. As a result, mechanical properties of the sintered bars rose, peaked and started to decline at an earlier mix time; that is, the decline in mechanical properties became apparent after two hours of mixing in the double-cone blender rather than after six hours in the smaller jars.

A further comparison of the advantages of the powder mix of the invention over identical mixtures except for the use of conventional copper powder appears from the following test:

Bar samples of both the aforementioned copper flake mixture of the invention (six hours mixing time), and bar samples of the same mixture except for the use of conventional copper powder, were sintered and were then immediately quenched in room-temperature water upon emerging from a continuous belt sintering furnace. Bars containing the copper flake, when quenched from 550C. and after five days of ageing, developed an U.T.S. of 34,370 psi and an elongation of 4.0 percent. Bars made with the conventional copper powder reached a maximum U.T.S. of 32,000 psi with a 1.8 percent elongation.

I claim:

1. An aluminum-copper-magnesium powder mixture adapted to be compacted and sintered to an agehardenable product which comprises an intimate mixture of metal powders free from internally lubricating amount of any organic material and consisting essentially of aluminum powder, about 2 to 6 weight percent of non-leafing copper flake powder containing on its surface between about 0.03 to 0.05 weight percent of a leafing-type lubricant, and about 0.2 to 2 weight percent of magnesium in powder form.

2. A powder mixture according to claim 1 which has been physically mixed for a continuous period of at least /2 hour.

3. A powder mixture according to claim 1 which has been physically mixed for a continuous period of at least 1% hours.

4. A powder mixture according to claim 1 in which the copper flake powder is present in amount of about 4 weight percent and the magnesium is present in amount of about 0.6 weight percent.

l l l l 

2. A powder mixture according to claim 1 which has been physically mixed for a continuous period of at least 1/2 hour.
 3. A powder mixture according to claim 1 which has been physically mixed for a continuous period of at least 1 1/2 hours.
 4. A powder mixture according to claim 1 in which the copper flake powder is present in amount of about 4 weight percent and the magnesium is present in amount of about 0.6 weight percent. 